Example of applying the SOLID principles to clean a view model.
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Example of applying the SOLID principles to clean a view model.

Step through the phases of cleaning up a wizard view model. Each phase has its own branch, so you can see exactly what changed at each step.


The story begins with a wizard view model implemented using a traditional MVVM approach. The WizardViewModel has several pages:

  • TitleViewModel
  • FileViewModel
  • UrlViewModel
  • ReviewViewModel

Each of these view models inherits from a base class that implements INotifyPropertyChanged. The WizardViewModel uses RelayCommands to operate four buttons. It determines which view model to display when the user clicks Back and Next. It also determines when the user has finished entering all of the data, so that it can activate the Finish button.

The WizardViewModel calls the IUploadService to upload the podcast when the user clicks Finish. The TitleViewModel calls the GenreRepository to load its list of genres.

Notice how the WizardViewModel subscribes to the PropertyChanged event on its child pages so that it can update the preview page. It has to, because each page has a separate copy of the data. If it didn't use PropertyChanged, then it would have to use a message bus or something similar to achieve the same result.


When we transition to Assisticant, we change all of the mutable properties to Observables. This means that we don't have to keep separate copies of the data in each view model. Instead, they all reference the same Upload object, which eliminates the need to subscribe to changes to keep each other in sync. We get to delete that code from WizardViewModel.

The WizardViewModel also starts to take advantage of contention-based ICommand implementations. Instead of creating RelayCommands, it implements properties like CanGoBack to determine when the user can click the Back button. And it implements methods like Back to execute that command.


The WizardViewModel has far too many responsibilities. It projects the model into a form that can be bound to the view. But it also calls the IUploadService to upload the poscast. If the view changes, we have to change the view model. And if the service API changes, we also have to change the view model. The view model has two reasons to change.

We move this extra responsibility out of the view model and into the model. This makes things a little better, but it's not good enough. Now the model is taking on too much responsibility. And some of that responsibility is asynchronous, which the model is not equiped to handle. Notice the proliferation of async void methods.


Rather than just shifting responsibilities, let's introduce a whole new class to manage service calls. The ServiceManager takes on the responsibilities of calling services. It does so in a responsible asynchronous fashion. It keeps a busy indicator, and catches any exceptions that might happen.

To show the user the busy indicator and the exception, we added a MainViewModel. This gets injected an IServiceStatus interface, which the ServiceManager implements for us. Because this status is observable, we can data bind to these properties to see when a call is in progress, or when an error has occurred.


There are a couple of places where the view model exposes more than it should. Let's segregate those interfaces so that the client doesn't have access to any methods that it shouldn't use.

The GenreViewModel has so far been returning an ObservableCollection<Genre>. That means that a caller could add or remove objects from that collection, or change the name of one of the genres. This observable collection is in the GenreRepository itself. Let's nip that in the bud by first having the repository convert the collection to an ImmutableList<Genre> before returning it.

But that's not quite enough. We also want to limit the interface by not returning a full Genre object from the TitleViewModel. Let's wrap each one in a GenreViewModel that provides just the surface area that the view needs. Since names are read-only on this particular view, they should be getter-only properties.

We'll take advantage of this opportunity to order the list of genres by name. We do this by adding a single orderby clause to the linq statement.


Now we can tackle the elephant in the room. The WizardViewModel is not open to extension and closed for modification. It knows far too much about the pages that it contains. It orchestrates the navigation from page to page. Let's move some of that intelligence into the pages themselves.

Each page implements a new IPage interface, which exposes an Active property. In its implementation, it determines when that page is an active part of the navigation flow. The wizard view model will simply transition to the next or previous active page.

Next, we notice that the wizard view model is also quite knowledegable about when the user has filled in all of the necessary fields of the upload. Rather than owning that domain logic itself, we'll let it delegate to the Upload object.

Now if we need to add features to the upload, we can do so without modifying the WizardViewModel. It simply delegates to the pages to see if they are active for navigation, and to the Upload to see if it is complete. We can extend the behavior of the wizard without modifying this class.

At this point, we can also simplify our tests quite a bit. Before, we had to test the WizardViewModel, because it had all of the domain knowledge. That means that we also had to mock all of its dependencies. However, now that we've offloaded that domain knowledge, we can simply test the Upload. It has no dependencies, so it's really easy to test.


Over several iterations, we have cleaned up this view model and its children. We started with all of the data stored inside the view models. They had to do a lot of coordination to get the right behavior. Moving to Assisticant allowed us to shift that data into a shared domain object.

With that extra freedom, we gradually refactored to move responsibilities away from the wizard view model. We moved the responsibility of calling services to a service manager. And we moved the responsibilities of determining page navigation to the pages themselves. Finally, we moved the responsibility of determining when the domain object was completely filled out into the domain object where it belongs.

The result is a cleaner set of view models. These are easier to read, easier to test, and easier to modify.